1. Field of the Invention
The present invention relates to a method for producing nickel powder from a solution containing a nickel ammine sulfate complex using fine nickel powder.
2. Description of the Related Art
Example of known methods for producing nickel powder which is expected to be used as a material for conductive paste and positive electrode active material of a nickel-hydrogen battery and the like include a method shown in Japanese Patent Laid-Open No. 2005-194156. The production method disclosed in Japanese Patent Laid-Open No. 2005-194156 is a method for producing nickel hydroxide powder, including: treating, under high temperatures and high pressures, a solution containing a nickel ammine complex produced to cause a reaction represented by the following formula (1) to precipitate nickel hydroxide particles.[Formula 1][Ni(NH3)x]2++xH2ONi2++xNH4++xOH−Ni2++2OH−→Ni(OH)2↓  (1)
Nickel powder can be obtained when the nickel hydroxide is reduced with a reducing agent. Although various reducing agents can be used, hydrogen gas is industrially inexpensive and has been widely used.
Further, POWDER METALLURGY, 1958, No. ½, P. 40-52 describes a process for producing nickel powder in Sherritt Gordon Inc.
This production method includes mixing a complexing agent with a nickel sulfate aqueous solution to form a solution containing a nickel ammine complex, putting the solution in a pressurized vessel such as an autoclave, heating the solution to about 150 to 250° C., and blowing hydrogen gas into the solution, in which the nickel ammine complex salt is reduced by hydrogen to obtain nickel powder.
However, the method called a complexing reduction method shown in POWDER METALLURGY, 1958, No. ½, P. 40-52 has a problem that when hydrogen gas is blown, a large amount of fine nickel powder will be nonuniformly produced in many cases if seed crystals serving as nuclei are not present, and as a result, it will be difficult to obtain nickel powder having a predetermined particle size, which makes it difficult to obtain uniform quality.
Further, the method also has a problem that since fine nickel precipitates, like scaling, on the inner wall of the pressurized vessel and on the surface of equipment such as a stirrer, the time and efforts of maintenance of facilities increase, and the rate of recovery of products decreases. Thus, this method is not preferred.
In order to avoid such a state, it is known that such a state can be suppressed by adding seed crystals to the solution and blowing a reducing agent into the resulting mixture. When a reducing agent is blown into the nickel ammine complex solution to thereby start the precipitation of nickel, the addition of the seed crystals advances the growth of nickel using the seed crystals as nuclei and can suppress the occurring of nonuniform precipitation of fine nickel on the surface of vessels and equipment as described above.
In many cases, fine crystals of the same product class are used as seed crystals, or a part of products is processed by crushing or the like and used. On the other hand, it is known that the proportion and the shape of seed crystals added also influence the quality of products.
However, as described above, when a part of products is used, the processing will take time and efforts and may be a factor of cost increase. Further, there has been a problem that quality is not stabilized caused, for example, by the variation in the shape of seed crystals obtained by processing.
Further, a material that has a uniform and fine shape and is easily industrially obtained, for example, iron powder, is also often used as seed crystals.
However, if iron powder, for example, is used as seed crystals as used in the process of POWDER METALLURGY, 1958, No. ½, P. 40-52 in order to obtain nickel powder, the iron powder will influence the quality of products. Therefore, the application of the products will be limited, and such a method is not preferred.
Thus, in order to stabilize the quality of products and reduce production cost, a production method has been desired in which seed crystals of the same type as products are used, and at the same time, the amount of the seed crystals added is suppressed as much as possible.
In such a situation, in order to solve a problem of a reduction in productivity and an increase in cost in a production method including adding nickel seed crystals to a solution containing a nickel ammine complex and subjecting the resulting mixture to hydrogen reduction under high temperatures and high pressures to obtain nickel powder, the present invention provides a production method that reduces the amount of seed crystals used and maintains the quality of nickel powder.